1. Field of the Invention
The invention relates generally to earth-boring drill bits. More particularly, the invention relates to increasing the reliability and manufacturing efficiency of earth-boring drill bits. Still moreh particularly, the invention relates to maintaining desired carbon percentages in a material during heat treatment processes performed subsequent to carburization.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. A typical earth-boring bit includes one or more rotatable cone cutters that perform their cutting function due to the rolling movement of the cone cutters acting against the formation material. The cone cutters roll and slide upon the bottom of the borehole as the drillstring and bit are rotated, the cone cutters thereby engaging and disintegrating the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The cones are mounted on bearing pin shafts (also called journal shafts or journal pins) that extend downwardly and inwardly from the journal segment legs. As the bit is rotated, cutter elements or teeth that extend from the cone cutters remove chips of formation material (“cuttings” or “drilled solids”) which are carried upward and out of the borehole by the flow of drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location which, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes—which in oil and gas well drilling may be miles long—must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. The amount of time required to make a round trip for replacing a bit is essentially lost time and lost productivity from drilling operations. It is therefore advantageous to employ drill bits that will be durable enough to drill for a substantial period of time with acceptable rates of penetration (ROP) so as to minimize the number of “trips” and the associated lost productivity.
One cause of bit failure arises from the severe wear or damage that may occur to the bearing surfaces on which the cone cutters are mounted. It is therefore desirable to maintain a hard surface on the journal shaft or pin to minimize wear and damage, and thereby minimize the need to trip the drill string. One method used to increase the surface hardness of the journal shaft is to carburize the area. Carburization is well known in the art, and generally comprises heating the material to an elevated temperature (approximately 1750 degrees Fahrenheit) in a carbon-rich environment (approximately 0.6% to 0.9% carbon, depending on the material being treated). This allows carbon to diffuse into the surface, thereby increasing the hardness of the material.
While carburization provides good surface hardness, it does not produce a material that has other desirable mechanical properties, such as ductility. In order to improve the mechanical properties of the material used to manufacture earth-boring drill bits, it is common to “heat treat” the material. This involves heating the material to a temperature of approximately 1500 degrees Fahrenheit, and then rapidly cooling, as by quenching. This has the effect of increasing the hardness of all of the material, not just the surface, as is accomplished via carburization. The final heat treatment step typically conducted in the manufacture of an earth-boring drill bit is to temper the material at a temperature of approximately 400 degrees Fahrenheit to increase the toughness and ductility of the material.
During the heat treatment steps performed subsequent to carburization, it is desirable to maintain the high carbon concentrations in the carburized areas to provide improved wear characteristics. It is also desirable during these steps to prevent excess carbon from diffusing into the areas that were not carburized because excess carbon in these areas can decrease the ductility of the material and lead to reduced fatigue properties and increased likelihood of the material developing cracks.
In the prior art, the process of carburization and subsequent heat treatment is therefore performed in the following basic steps. First, the portions of the drill leg that are not intended to be carburized are painted, while the areas that will be carburized (i.e. the journal pin surfaces) are left exposed. The drill leg is then subjected to the carburization process by exposing the leg to an elevated temperature in a carbon-rich environment. After carburization, any defects or breaches in the painted areas of the drill leg are repainted, while the carburized areas are still left exposed. Finally, the drill leg is heat treated at an elevated temperature in an environment having a carbon percentage that is substantially the same as the carburization environment. This carbon-rich environment is again employed in order to prevent the carbon that diffused into the surface of the carburized surfaces during carburization from “reversing” itself and diffusing out of the surface and into the atmosphere surrounding the part. With respect to the percentage of carbon, this environment surrounding the part during heat treating is known as “neutral to the case”.
As mentioned, by performing the subsequent heat treatment in such an environment, there is a reduced tendency for the carbon to diffuse from the journal pin into the environment. One problem with using the high carbon environment in this conventional process is that the coating or paint on the painted areas of the drill leg must be maintained so that no areas of the base material are exposed. If any portion of the base material is exposed to the carbon-rich environment during the carburization or heat treatment processes, excess carbon will be diffused into the exposed portion. Such diffusion will result in the exposed area's mechanical properties, such as ductility and fatigue strength, being lowered. In turn, such a resultant decrease in mechanical strength increases the likelihood that drill leg will break during operation, resulting in increased downtime and operating costs.
The intricate shape of the drill leg increases the likelihood that a portion of the drill leg will be unintentionally exposed during the carburization or heat treatment processes. Furthermore, the handling and transporting of the drill leg during the carburization and heat treatment steps increases the possibility of breaching the protective coating or paint. In addition, the time period between the carburization and heat treatment can be significant, further increasing the likelihood that a portion of the painted area will be exposed during handling, for example.
Thus, the embodiments of the present invention are directed toward methods and apparatus for maintaining carbon concentrations in the carburized areas of the drill bit during subsequent heat treatment processes. Furthermore, embodiments of the present invention are directed towards methods and apparatus to prevent carbon diffusion into those non-carburized areas of the drill bit during subsequent heat treatment processes.